The present invention relates to an ultrasonic imaging system by which both orthographic images and tomographic images of high resolution can be obtained, in particular, relates to such an apparatus employing only a few transducers.
Some of the application fields of the present invention are the medical field, sonar, and non-destructive inspection.
There have been two kinds of conventional ultrasonic imaging systems having a three-dimensional imaging function capable of obtaining orthographic and tomographic images simultaneously. The first one employs an acoustic lens and a plane transducer array for both transmission and reception, and scans an object by sequentially switching on and off the transducer elements in the array. The second one employs a variable delay line and plane transducers for transmission and reception, and scans electronically the directions and the focal distance of the transmitting and receiving beams (for instance, "Acoustic Imaging" by G. Wade, Plenum Press (1976), PP171-181).
However, the prior ultrasonic imaging systems have the disadvantage that a plane transducer array must have almost the same number of elements as the number of picture cells of an object. Therefore, the number of transducer elements becomes enormous when an image of high resolution must be obtained, thus, the system is almost impracticable. Further, the first prior art has the disadvantage that dynamic focusing is not possible for tomographic imaging since the focal distance is fixed, although the signal processing is simple since the image is obtained by the image formation of the acoustic lens. The impossibility of dynamic focusing restricts the bearing resolving power. The second prior art can perform dynamic focusing, but the signal processing is complex, the size of the circuitry is large, and in fact, the second prior art is almost impracticable.